1. Field of the Invention
The present invention relates generally to optical transmission, and more particularly to methods and apparatus for receiving phase shift keyed (PSK) optical signals.
2. Description of Related Art
The use of PSK modulation, and in particular, differential phase shift keying (DPSK) has been shown to provide improved performance as compared to other modulation formats. DPSK has been shown to provide a 3-dB enhanced receiver sensitivity over conventionally used on/off keying (OOK) modulation formats. Such improved receiver sensitivity allows systems employing DPSK modulation to transmit over longer distances without signal regeneration.
DPSK signal transmission has also been found to be more robust to narrowband optical filtering as compared to other modulation formats. Such characteristics make DPSK modulation a desirable format for optically-routed networks that incorporate multiple optical add/drop multiplexers (OADM) or optical crossconnects (OXC).
In order to fully exploit the benefits of phase shift keyed (PSK) modulation techniques, PSK optical receivers are typically built using an optical delay interferometer followed by a balanced photoreceiver. Prior art teachings suggest that optimum receiver performance is obtained if the balanced receiver is perfectly amplitude-balanced. However, for systems incorporating one or more optical filter devices (e.g. OADMs or OXCs), it has been found that use of perfectly amplitude-balanced reception of PSK signals results in degraded receiver performance. Specifically, the performance is degraded in terms of the optical signal-to-noise ratio (OSNR) that is required to achieve a desired bit-error-rate (BER) upon detection of a PSK signal.